The present invention relates to a variable geometry turbine incorporating a displaceable turbine inlet passage sidewall.
U.S. Pat. No. 5,522,697 describes a known variable geometry turbine in which a turbine wheel is mounted to rotate about a pre-determined axis within a housing. An inlet passage to the turbine wheel is defined between a fixed wall of the housing and a sidewall which is displaceable relative to the fixed wall in order to control the width of an inlet passage. The sidewall is supported on rods extending parallel to the wheel rotation axis, and the rods are axially displaced relative to the housing so as to control the position adopted by the sidewall.
The rods are displaced by a pneumatic actuator mounted on the outside of the housing, the pneumatic actuator driving a piston. The actuator piston is coupled to a lever extending from a shaft pivotally supported by the housing such that displacement of the lever causes the shaft to turn. A yoke having two spaced apart arms is mounted on the shaft in a cavity defined within the housing. The end of each arm of the yoke is received in a slot in a respective sidewall support rod. Displacement of the actuator piston causes the arms to pivot and to drive the sidewall in the axial direction as a result of the interengagement between the arms and the sidewall support rods.
The known variable geometry turbine exhibits various disadvantageous features. In particular, pneumatic actuators typically incorporate an elustomeric diaphragm which is prone to failure, particularly in the temperature, piston stroke and pressure environment associated with variable geometry turbines. The shaft which supports the yoke is exposed to high temperatures but cannot be readily lubricated and therefore wear can arise. Furthermore, the engagement of the levers with the rods is of a sliding nature and although it is known to incorporate wear resistant materials, e.g. ceramics, in such assemblies, wear can still be a problem. Finally, mounting a pneumatic actuator outside the housing increases the overall size of the assembly which can be a critical factor in some applications.
It is an object of the present invention to obviate or mitigate one or more of the problems outlined above.
According to the present invention, there is provided a variable geometry turbine comprising a housing, a turbine wheel mounted to rotate about a pre-determined axis within the housing, a sidewall which is displaceable relative to the housing to control the width of a gas inlet passage defined adjacent the wheel between a first surface defined by the sidewall and a second surface defined by the housing, and displacement control means for controlling displacement of the sidewall relative to the housing, wherein the housing defines at least one chamber forming a cylinder which receives a piston defined by the sidewall, the sidewall is displaced as a result of displacement of the piston, and the displacement control means comprise means for controlling the pressure within the said at least one chamber to control the position of the sidewall relative to the housing.
The piston and cylinder may be annular.
The sidewall may be supported on guide rods extending parallel to the wheel rotation axis. The sidewall and guide rod assembly may be biased away from or towards the second surface by at least one spring. Each rod may be biased by one or more springs. The spring or springs may have a variable spring rate such that the rate of change of spring force with gas inlet passage width increases as the sidewall approaches the second surface. For example, each guide rod may be acted upon by two springs, one spring being compressed only when the sidewall approaches the housing surface.